Power Line Filters

EMI filters are used in power grids to suppress the noise is known as EMI power line filters. Currently, electronic devices or equipment mostly use SMPS (Switching Mode Power Supply) and fast digital circuits. When these devices or equipment operate in normal conditions, they generate high-frequency voltage and current. It is nearly impossible to meet the requirements of the EMC (Electromagnetic Compatibility) standards without using the power line filter.

 Power line filters mainly perform two primary functions. Firstly, it prevents the high-frequency signals (that get generated within the device) from reaching the input power line. And secondly, it prevents the high-frequency signals (produced on the AC power distribution system) from entering into the device or equipment.

The efficiency of a power line filter does not only depend on the electrical design of the filter but it equally depends on where the filter is mounted, how it is mounted, and how leads are connected to the filter. Following are the three common issues associated with the mounting of the power line filter that significantly decreases its effectiveness:

1. The power line filter mounted far from the point where the power line enters the enclosure.

In this case, the exposed power line or antenna can bring the noise from the electric and magnetic fields inside the enclosure. Hence, the power line should be mounted at/close to the point where the AC power line enters the enclosure. This helps to prevent the coupling between the electromagnetic field and the filtered power line. The metal enclosure further blocks any capacitive coupling from the filter input cable and the filtered power line.

2. Wires between the cover of the power line filter and the enclosure.

The wire grounding the filter to the enclosure has a high inductance due to which the effectiveness of the Y-capacitors in the filter decreases. In this case, the manufacturer should assemble the Y-capacitors in such a way that the connection with the cover shows minimum inductance. The metal case of the power line filter must make direct contact with the device enclosure to eliminate any additional inductance in series with the internal Y-capacitors. 

3. Capacitive coupling between the noisy power supply to the wiring of the power line filter and the AC power line.

Wires between the power line filter and the power supply should be connected close to the enclosure to minimize any noise pickup. Never connect the input leads of the filter close to the output DC power leads. This will result in maximizing the parasitic capacitance coupling. In addition to this, ensure to keep the input leads away from any signal cables (mainly digital cables) and never connect them near or over a digital logic PCB (Printed Circuit Board).

 You can also mount the power supply close to the power line filter to resolve this issue.

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